1. Field of the Invention
The present invention relates to a lens barrel mounted on a camera, and more particularly to a cam tube of the lens barrel, which is provided with a cam groove for driving the component members of the lens barrel through a cam.
2. Description of Related Art
In a case where a cam tube having a cam groove on the inner circumferential side thereof is to be formed with a synthetic resin such as a plastic material by injection molding, a mold for forming the inner wall of the cam tube, which is divided into a plurality of parts, for example, six parts, is used in many cases.
FIG. 5 schematically shows the arrangement of inner-diameter slide pieces (molds) to be used for forming the inner cicumferential surface of a cam tube. The inner-diameter slide pieces are, in fact, composed of a total of six pieces, i.e., three large slide pieces and three small slide pieces. In FIG. 5, reference charcters D-A and D-B denote the large slide pieces, and reference character D-C denotes the small slide piece, while the remaining one large slide piece and two small slide pieces are omitted from the illustration. Each of these slide pieces is provided with a projection D-1-T for molding a cam groove for a first lens group and a projection D-2-T for molding a cam group for a second lens group.
When a plastic material is injected to the outer circumferential side of the slide pieces and is then cooled, a mold removing process is performed by moving the small slide piece D-C first toward an optical axis (the center axis of a cylindrical shape) to leave a gap space there and then moving the large slide pieces D-A and D-B toward the optical axis utilizing the gap space formed by the movement of the small slide piece D-C.
All of the slide pieces are thus removed by pulling them off toward the optical axis. To be exact, however, it is only a middle part of every slide piece, as viewed in the circumferential direction thereof, that is pulled off toward the optical axis. The end parts of each slide piece located away from the middle part in the circumferential direction move in the direction parallel with the direction toward the optical axis, instead of moving toward the optical axis. Therefore, the parts of the cam groove to be molded by the circumferential end parts of the large slide pieces D-A and D-B, particularly, such parts that have a large lead, must be formed to have their cam-groove side wall surfaces tapered at a large angle. In other words, these parts must be formed to have such tapered surfaces that are widely open with respect to the direction toward the center line of the cam tube.
Heretofore, the taper angle of side wall surfaces of a cam groove which is considered to be necessary for the two end parts of the large slide piece has been decided according to the allocation of the inner-diameter slide pieces. Further, all the areas of the cam groove are formed at a maximum necessary taper angle. If the taper angle is obtuse, cam followers tend to come off the cam groove when a strong impact is inflicted on the lens barrel in the direction of a photo-taking optical axis.
According to an arrangement disclosed in Japanese Laid-Open Patent Application No. Hei 7-43581, to increase the strength of the edges of a cam groove formed in a cam tube, the taper angle of a side wall surface on one side of the cam groove is gradually increased while another side wall surface on the other side of the cam groove is formed to have a small taper angle.
FIG. 6 shows the arrangement of a lens barrel disclosed in the above Japanese Laid-Open Patent Application No. Hei 7-43581. As shown in FIG. 6, the lens barrel is composed of a rectilinear motion member 101 which has penetrating helical cam grooves 101a, and a rotary member 102 which has cam followers 102a press-fitted therein. The movement of the rotary member 102, i.e., the cam followers 102a, in the direction of the groove width of each of the cam grooves 101a is arranged to be restrained by the groove width of the helical cam groove 101a. 
Then, in the helical cam groove 101a, there are continuously formed a part where the rotary member 102 is cam-driven within a photo-taking movable range in which the rotary member 102 is movable at the time of photo-taking (a zoom-and-focusing range) and another part where the rotary member 102 is cam-driven within a non-photo-taking movable range in which the rotary member 102 is movable when no photo-taking operation is performed (a stowing range).
However, according to the arrangement disclosed in the above Japanese Laid-Open Patent Application No. Hei 7-43581, the wall surfaces on two sides of the helical cam groove 101a have different taper angles not only at the cam driving part for the rotary member 102 within the non-photo-taking movable range but also at the cam driving part within the photo-taking movable range.
Therefore, it is necessary, for preventing an adverse effect on the focusing accuracy, not to use the two side wall surfaces for preventing the rotary member 102 from moving in the direction of the groove width of the cam groove but to use only the width of the cam groove for preventing the movement in the direction of the groove width of the cam groove. In order to prevent the movement of the rotary member 102 in the direction of the groove width solely by the width of the cam groove, the cam groove must be formed in a penetrating shape as a through groove. The arrangement as mentioned above, therefore, cannot be used in cases where it is impossible to have the cam groove formed as such a through groove.
Further, with a cam groove formed in the inner wall of the cam tube, the taper angle of the side wall surface of the cam groove considered to be necessary at the two end parts of the large slide piece is decided according to the allocation of the inner-diameter slide pieces, as mentioned above. A taper angle of the side wall surface of the cam groove considered to be necessary at the middle part of the inner-diameter slide piece is, therefore, smaller than the taper angle considered to be necessary at the two ends of the inner-diameter slide piece. However, the cam groove is formed to have all side wall surfaces at a certain fixed taper angle. Therefore, the taper angle which is considered necessary only for the end parts of the inner-diameter slide piece is applied to the whole cam groove. As a result, the taper angle of the side wall surfaces is too large for the cam groove as a whole. Hence, the cam follower is apt to come off the cam groove when the lens barrel happens to receive an external force.
It is an object of the invention to provide a lens barrel including a cam tube arranged to have a non-penetrating cam groove, which is formed on an inner circumferential side of the cam tube in such a way as to make the taper angle of side wall surfaces of the cam groove as small as possible.
To attain the above object, in accordance with an aspect of the invention, there is provided a lens barrel, which comprises a cam tube having a cam groove formed on an inner circumferential side thereof, the cam groove having a first cam groove area over which a lens unit slides when the lens barrel is used for photo-taking and a second cam groove area over which the lens unit slides when the lens barrel is not used for photo-taking, wherein a taper angle of a side wall surface of the first cam groove area differs from a taper angle of a side wall surface of the second cam groove area.
Further, in the lens barrel, the taper angle of the side wall surface of the second cam groove area is larger than the taper angle of the side wall surface of the first cam groove area.
Further, in the lens barrel, of both side wall surfaces of the second cam groove area, a taper angle of one side wall surface differs from a taper angle of the other side wall surface.
Further, in the lens barrel, the first cam groove area is a cam groove for varying a focal length.
Further, in the lens barrel, the second cam groove area is a cam groove for stowing the lens unit in a camera body.
Further, there is provided a camera comprising the above lens barrel.
These and further objects and features of the invention will become apparent from the following detailed description of a preferred embodiment thereof taken in connection with the accompanying drawings.